This invention relates to a class BC power amplifier which can be used in amplifying devices for which a minimum volume and a low weight, prime cost and electric energy consumption per unit useful power are of primary importance. Such are, for example, the amplifying devices for installations of wired radio units in which the use of an output transformer is obligatory, orchestra amplifiers which may be without output transformer (transformer-less circuit), portable devices comprising low-frequency amplifiers, etc.
A known class BC power ampifier is built on the basis of a push-pull circuit. It comprises a symmetrical push-pull preamplifier, the two outputs of which are connected to the two branches of the amplifier, the one branch being connected to the positive poles of a pair of supply sources, while the other branch is connected to the negative poles of the supply sources, both branches being completely symmetrical, respective transistors having opposite conductivity. The one output of the symmetrical push-pull preamplifier is connected to the base of a saturated n-p-n transistor whose collector is connected, by means of a commutating diode, in pass direction with respect to its collector current, to the positive pole of a low-voltage supply source with a grounded center point. The emitter of the saturated transistor is connected through a series combination of an emitter resistor for the saturated transistor and an emitter resistor for a comparative transistor, to ground. The emitter of the saturated transistor also is connected directly to the base of a p-n-p transistor of the power stage operating in class B, whose emitter is both connected to the collector of the saturated n-p-n transistor and to the collector of a p-n-p transistor of the power stage operating in class C, whose emitter is connected directly to the positive pole of a high-voltage supply source with a grounded center point. The collector of the p-n-p transistor of the power stage operating in class B is connected through the load to ground and, by means of a protective diode in pass direction with respect to the base current, to the base of the comparative transistor, whose emitter is connected to the common point of the emitter resistors for the saturated and the comparative transistors, while the collector of the comparative transistor is connected directly to the base of the p-n-p transistor of the power stage operating in class C.
When a signal is received in the input of the amplifier with a level up to about the ratio of the low-voltage to the high-voltage source multiplied by the nominal input level, only the class B amplifier is actuated which is supplied by the low-voltage source. With the increase of the momentary value of the input signal above the aforementioned level, there are created conditions for rendering conductive the corresponding transistor of the amplifier operating in class C, which is supplied by the high-voltage source. The collector current of the transistors closes its circuit through the load to ground.
A drawback of the aforedescribed circuit lies in that the efficiency in the whole amplitude range and particularly in the case of weaker signals is relatively low.
There are also known other class BC power amplifiers in which the commutation is effected by Zener diode. Their drawback lies in that in the vicinity of the interval of commutation of the sources, the efficiency is considerably lowered.